Photosystem II (PSII) is a large membrane protein complex in the thylakoid membrane of plants and cyanobacteria [1-4]. It utilizes light energy to oxidize water on the lumen side and to reduce plastoquinone (PQ) on the stromal side of the thylakoid membrane [1-4]. However, photosystem II (PSII) is the major target for the light-induced damage of the photosynthetic apparatus under excess light conditions [5,6]. To avoid this problem, photosynthetic organisms have developed several distinct and sophisticated photo-protective mechanisms to ensure their survival under excess light conditions [5,6]. In higher plants, excess light energy is dissipated in PSII light-harvesting antenna (LHCII) as heat that reduces the amount of excitation energy reaching the reaction center and protects PSII from photo-damage. This process (also called nonphotochemical fluorescence quenching; NPQ) involves the activation of the xanthophyll cycle and the conformational change of LHCII which are controlled by the formation of ΔpH in the lumen of PSII [5,6]. In contrast, cyanobacteria use phycobilisomes as antenna to capture sunlight. A great number of cyanobacteria develop a distinct blue-green-light-induced NPQ mechanism to cope with high-light stress [8-10]. When exposed to high intensity of white light or blue light, a soluble orange carotenoid protein (OCP) will undergo photo-conversion into the active red form. The red form of OCP is able to interact with the allophycocyanin (APC) core of the phycobilisome and induce the NPQ effect which dissipates excess excitation energy on the phycobilisome as heat, thereby protecting PSII reaction centers against photodamage (7-17). During this process, the OCP acts as the light sensor, the signal propagator and the energy quencher. When the intensity of the blue light or white light is decreased, the binding between the red form of OCP and APC core of the phycobilisome is released with the help of fluorescence recovery protein (FRP). The FRP interacts with the C-terminal domain of the red form of OCP and accelerates its conversion to orange form and detach from the phycobilisome to stop the NPQ process (14). In contrast to the photoprotective mechanisms in higher plants and algae, up to now there is no evidence for the feed-back regulation of PSII in the OCP-mediated photoprotective mechanism in cyanobacteria.
The 2.9 Å resolution x-ray crystallographic structural models of PSII from the cyanobacteria Thermosynechococcus elongatus revealed a newly discovered PQ molecule (QC) and its diffusion channel [3, 18]. The head-group QC molecule locates in a very hydrophobic cavity surrounded by the tails of lipids, QB and a Car molecule; the tail of QC molecule situates in a hydrophobic channel, surrounded by the trans-membrane helixs of cytochrome (Cyt) b559 α and β subunits (encoded by psbE and psbF genes, respectively) and PsbJ, which is open toward the internal space of thylakoid membranes [3, 18]. The occupancy of this QC site by PQ (or PQH2) was proposed to be involved in exchange of PQ/PQH2 on the QB site from the pool [3, 18] or to modulate the redox potential and the reactivity of Cyt b559 [3, 18-20, also see ref 21 for a different view]. In addition, the QC site was also proposed as the catalytic site for PQH2 oxidase activity of Cyt b559 [22-29]. However, occupancy of the QC site was not observed in the recent 1.9 Å resolution crystal structural models of PSII from T. vulcanus [2]. The function of QC remains elusive.
WO 2012/092033 describes enhancement of biomass production in a photosynthetic microorganism by disruption of NPQ process via disrupting the production of at least one carotenoid and/or reducing the expression of at least one carotenoid binding protein, and suggests that reduction of NPQ allows a higher proportion of photons to provide energy for photochemistry and biochemical pathways that generate biomass. However, it has been reported that cyanobacterial photoactive orange carotenoid protein has new function in singlet oxygen quencher in thylakoid membrane in addition to energy quencher function (Sedoud et al, plant cell (2014) 26, 1781-1791). Therefore, disruption of NPQ process via disrupting the production of at least one carotenoid and/or reducing the expression of at least one carotenoid binding protein in cyanobacteria may lead to susceptible photoinhibition, which is light-induced reduction in the photosynthetic capacity, due to the disruption of the dual-photoprotective functions of orange carotenoid protein (plant cell (2006)).